The manufacture of fluorine by electrolysis of mixtures of fluorides is well known, the fluorine being derived, for example, from mixtures of an alkali metal fluoride and hydrogen fluorides. Systems of this kind are disclosed for example in U.S. Pat. Nos. 3,773,644 and 4,139,447 and in British Pat. No. 852,369. It is also known in such processes to use cells having anodes of carbon or graphite, the cathode being of mild steel or other metal resistant to the action of the electrolyte. Hydrogen is evolved at the cathode and fluorine, with perhaps varying amounts of oxygen and other impurities, at the anode. Also as mixtures of hydrogen and fluorine give rise to violent explosions, such fluorine cells customarily have a diaphragm or partition, also referred to as a "skirt" designed to prevent mixing of the gases evolved at the two electrodes. In some cells this diaphragm or partition extends downward in the interelectrode space for a distance equal to or even greater than that of the downward extension of the electrodes. In other fluorine cells, for example, and as disclosed in British Pat. No. 852,369, a barrier, impervious to gases, extends downwards for a short distance only into the interelectrode space.
It is recognized in prior art systems that the greater the spacing between the electrodes, the greater must be the potential applied and the energy consumed to electrolyze a given amount of material. Therefore it is desirable to diminish the interelectrode space as far as is commensurate with safety. Nevertheless, in general (except in certain cells with porous or gas permeable carbon anodes) it has not been possible in the prior art to safely diminish the distance between anode and cathode (hereinafter termed the electrode separation) or the distance between anode and gas barrier (hereinafter termed the anode gap) below certain limiting values. As stated in British Pat. No. 852,369, for instance, as the electrodes extend further downward into the electrolyte below the bottom of the gas barrier, the interelectrode spacing must be increased. A minimum is prescribed for safe working such that when the electrodes extend to 8 inches below the gas barrier, the electrode separation should not be less than 25/8 inches (6.65 cm) nor the anode gap less than 1 inch (2.54 cm). The corresponding values when the electrodes are extended to 36 inches (91 cm) below the barrer are 43/4 inches (12 cm) and 1 11/16 inches (4.3 cm). However, if a special louvered cathode is used, the figures for the electrode separations appropriate to these depths of 8 inches (20.3 cm) and 36 inches (91 cm) may be diminished to 21/4 inches (5.7 cm) and 3 15/16 inches (10 cm), respectively. However, as noted in said British Patent, these are prescribed as limiting minimum values if anodic current density does not exceed 0.15 .ANG./cm.sup.2.
By use of the anode of the invention, in which passages for the flow of gases are provided, high current densities are possible. Such current densities are even higher than those disclosed as "critical current densities" in U.S. Pat. No. 4,312,718.
Current density is determined with reference to that portion of the anode surface which is directly opposite to the cathode.
A number of reasons exist for the inefficiency of prior art electrolytic cells for fluorine production. One reason, for example, is the low productivity which is due to low anodic current density and comparatively small anode length and/or an undesirably large distance between the anode and cathode. Also, because of low anodic current density, these prior art cells necessitate high equipment cost and high capital cost outlays.
It is thus apparent that a need exists for an improved cell configuration which enhances the economy as well as the efficiency of systems for the electrolytic production of fluorine.